N = N0 e-leff t

at t = 2 hours, N = 0.80 N0

0.80 = e-leff 2

l eff = ln0.80/(-2 hr) = 0.1116 h-1
 

Also; l p = ln2/Tp = ln2/(4X24 hr) = 7.22X10-3 h-1
 

l eff = lp + l b

0.1116 = 7.22X10-3 + l b

l b = 0.1046 h-1

Tb = ln2/l b = ln2/(0.1046 h-1) = 6.64 h = 0.28 d

Answer is C

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